In order to apply nitride semiconductor light emitting devices (referred to as LED (Light Emitting Diode), hereinafter) to white light lighting apparatuses, it is inevitable to achieve higher optical efficiency and higher output performance. For this reason, improvements of internal quantum efficiency, light extraction efficiency, etc. have been advanced by research and development of crystal and device structures.
For the crystal growth of a nitride semiconductor, a sapphire substrate is often used, because it is cheap and stable at a high temperature, and it can grow with high crystalline quality by means of low-temperature buffer. The sapphire substrate is an insulator with no electrical conductivity and low thermal conductivity. For this reason, an electrode cannot be formed on the rear side of the sapphire substrate.
Accordingly, a light emitting device has been proposed in which an LED-structured crystal grown on a sapphire substrate is transferred onto another support substrate made of silicon, cupper, gold, etc. In the transfer process, laser lift-off is performed to peel off the sapphire substrate.
In the laser lift-off, a technique has been proposed in order to prevent micro cracks from being generated in a multi-layered nitride semiconductor layer. In the technique, a laser beam is radiated from above onto a transparent crystal wafer to be peeled off to form a groove at a depth of reaching the multi-layered nitride semiconductor layer. And then, a laser beam is radiated again from above onto the transparent crystal wafer to peel off the wafer.
The reason for forming the groove in the multi-layered nitride semiconductor layer before the peeling-off step is to release nitrogen gas through the groove. The nitrogen gas is generated when a part of the multi-layered nitride semiconductor layer is decomposed when irradiated with a laser beam in the peeling-off step.
Another technique has been proposed in which a groove is formed on a sapphire substrate at the location to be irradiated with a laser beam without completely peeling off a light emitting structure in a first separation step. The sapphire substrate is then irradiated with a laser beam to be peeled off in a second separation step.
The reason for providing the first separation step is to protect a conductive bonding layer or a permanent conductive substrate at the lower side of the light emitting structure from being damaged.
In the above two techniques, a laser beam is radiated several times in the laser lift-off step.